1. Field of the Invention
The present invention relates generally to an optical fiber testing apparatus for locating a possible fault such as breakage in an optical fiber by inputting a coherent light beam to the optical fiber at one end thereof and detecting a return light beam by using an optical heterodyne detection.
2. Description of Related Art
For a better understanding of the present invention, description will be made of an optical fiber testing apparatus known heretofore by reference to FIG. 4.
In this figure, reference numeral 21 denotes a photodetection circuit, 22 denotes an amplifier, 23 denotes a filter, 24 denotes a detector, 25 denotes an analogue-to-digital (A/D) converter, 26 denotes a data processing unit and 27 denotes a display unit.
For testing an optical fiber (not shown in FIG. 4), a coherent light beam emitted by a laser light source of a frequency f (not shown either) is inputted to the optical fiber. A return light beam reflected back from the optical fiber and having a frequency of f+.DELTA.f is detected by the photodetection circuit 21 through optical heterodyning technique by using a reference light beam of a frequency f derived from the coherent light source. The output of the photodetection circuit 21 is amplified by the amplifier 22, the output of which in turn is supplied to the filter 13 to undergoes limitation in the frequency band and in order to improve the SN ratio. The detector 24 performs an envelope detection on the output signal of the filter 13 which is modulated in amplitude with a carrier frequency of .DELTA.f, to thereby extract only the envelope signal. The output signal of the detector 24 is converted to digital data by the A/D converter 25. The data processing unit 26 averages the digital data supplied from the A/D converter 25 for improving the SN ratio. The display 27 displays the output of the data processing unit 26.
The coherent light type optical fiber testing apparatus is advantageous over a direct detection type optical fiber testing apparatus in that the former can enjoy a broader dynamic range than the latter. However, in the coherent light type optical fiber testing apparatus known heretofore in which the optical heterodyne detection in adopted, limitation is nevertheless imposed on the dynamic range due to the detection characteristic of the detector 24.
FIG. 5 is a view showing graphically a relation between the input voltage and the output voltage of the detector 24 shown in FIG. 4. Referring to FIG. 5, in a region where the slope is constant, i.e. region c, the detection efficiency is constant. It can be said that as this region is wider, the dynamic range is broader.
FIG. 6 shows a waveform obtained as the result of the optical fiber test when an ideal detector is employed as the detector 23, while FIG. 7 shows a corresponding waveform obtained when a conventional diode is employed as the detector.
In the hitherto known optical fiber testing apparatus in which the coherent light beam is used, gain of the amplifier 22 is changed in dependence on divided dynamic ranges of the detector 24 to perform several measurements for testing one optical fiber, the results of which are subsequently synthesized.
FIG. 8 shows waveforms obtained as the result of execution of three successive measurement steps, wherein the result of the first measurement is shown at a with that of the second measurement being shown at b. It the measurement step b, gain of the amplifier 22 is increased beyond that used in the measurement a by an amount corresponding to an allocated dynamic range. In the measurement step c, gain of the amplifier 22 is further increased beyond that employed in the measurement step b by a magnitude corresponding to the dynamic range allocated to this measurement.
FIG. 9 shows a waveform obtained as result of synthesization of the measured waveforms a, b and c.
As will now be understood from the above description made with reference to FIGS. 5 to 9, the prior art optical fiber testing apparatus requires several measurement steps for testing a single optical fiber, involving remarkable time consumption in the test. Further, additional processing is required for synthesizing a plurality of measured waveforms, to make expensive the optical fiber testing apparatus.